Chemical vapor deposition or CVD processes are widely utilized in semiconductor fabrication. Generally, in a CVD process, several constituent gases are mixed together in a reaction chamber containing a heated substrate, such as a semiconductor wafer. A chemical reaction occurs between the constituent gases close to or at an exposed, elevated temperature surface of the wafer in the chamber and produces a material film or layer on the wafer surface. The deposition of the layer on the wafer and the type of layer which is formed depends upon the constituent gases chosen and the chemical reaction between those gases, and may, for example, be titanium, titanium nitride, tungsten, tungsten silicide, etc.
The wafer receiving a material layer by CVD is supported in the reaction chamber by a suitable mount. The wafer mount conventionally has a planar support platform which supports the wafer from below to expose an upper surface of the wafer. The gas constituents of the CVD process require energy at the exposed upper surface of the wafer for proper chemical reaction and layer formation. Therefore, energy is often provided to the CVD process in the reaction chamber, and specifically, proximate the wafer to assist the chemical reaction. In one conventional CVD process, such energy is provided in the form of heat or thermal energy, which is applied to the reaction chamber itself or directly to the wafer being processed.
When applying heat to a wafer for one particular CVD process, the mount supporting the wafer within the CVD reaction chamber is a heated wafer chuck. The wafer to be processed contacts and is supported by the heated chuck, and heat or thermal energy is therefore applied directly to the wafer by conduction from the heated chuck. The walls of the CVD chamber in such a process are not heated, due to the direct heating of the wafer. Therefore, such a CVD process is commonly referred to as a "cold wall" CVD process.
One particular use for a cold wall, heated chuck CVD process is to apply tungsten silicide or a blanket film of tungsten onto a silicon wafer. For such depositions, silane, argon, and a tungsten-containing gas, such as WF.sub.6, are premixed and are then introduced into the reaction chamber. The thermal energy supplied to the wafer drives and sustains the necessary chemical reaction for forming the desired film. As mentioned, the heated chuck provides heat energy to accomplish the CVD process through the transfer of heat to the wafer.
The amount of thermal energy required for CVD processes makes it necessary to heat the chucks to very high temperatures, e.g., from 360.degree. C. up to 600.degree. C. Therefore, such CVD systems include chuck temperature control systems and components which operate the heated chuck. During the CVD process, the temperature of the chuck is precisely controlled. More specifically, a resistive thermal device (RTD) probe is thermally coupled to the chuck to monitor the temperature of the chuck as it is heated during a CVD coating process. RTD probes are elements which have an effective output electrical resistance which is directly proportional to the temperature of the probe. Such RTD probes are usually physically embedded inside of the body of the heated chuck whose temperature is to be monitored. RTD probes provide an indication, in the form of a measurable electrical resistance, of the temperature of the heated chuck. The chuck temperature control system then heats or cools the chuck, depending upon the RTD probe reading, to maintain it at a selected operational temperature.
One available CVD system utilizing thermal chucks and RTD probes is the Genus 8700 LPCVD system manufactured by GENUS of Sunnyvale, Calif. The GENUS system has six thermal or heated chucks made of Monel, a copper/nickel alloy, and each chuck utilizes a separate RTD probe. Each heated chuck is controlled by a separate temperature control system for applying heat to the chuck and monitoring the RTD probe. The individual RTD probes are embedded inside beryllium caps for insulation and protection inside of the heated chucks.
The high temperatures of the heated chuck and associated processing components make it imperative that the chuck temperature control system and RTD probes operate properly. A malfunctioning RTD probe or control system may cause overheating and thermal runaway of the heated chuck. The overheated chucks, in turn, may cause a meltdown of one or more of the chucks within the CVD system. Not only may such a meltdown cause failure of the entire CVD system and damage to the wafers being processed, but it may create safety concerns as well, due to the high temperatures involved.
In addition to thermal runaway, the calibration and verification of the operating status of the temperature control systems and RTD probes is also a concern. While malfunctioning RTD probes and control systems may not lead to overheating and meltdown, their operation may be sufficiently degraded such that the overall CVD process is hindered, or even prevented. Accordingly, it is necessary to calibrate and verify the operation of the control systems, and particularly to calibrate their associated RTD probes.
Conventionally, calibration and verification of a heated chuck temperature control system and RTD probe takes a substantial amount of time. A "black box" verification circuit must be specially built for a particular chuck temperature control system. The "black box" circuit must then be calibrated and interfaced to the chuck temperature control system each time the calibration of the system is performed. To do so, the RTD probe must be unplugged, which would shut the system off. However, it is desirable that the system be running when it is calibrated and verified.
Furthermore, currently available temperature monitoring systems for multiple heated CVD chucks are usually incorporated onto a single circuit board, making the multiple control systems difficult to isolate and analyze. Failure of the control system for a single heated chuck which is not corrected may then affect the entire system and the operation and monitoring of the other heated chucks. Therefore, it is desirable to immediately repair or calibrate each heated chuck control system which is malfunctioning.
Still further, the physical difficulty of accessing and isolating the various temperature control systems for verifying the operation of each, as well as calibrating each, makes the process slow, tedious, and therefore, expensive from a maintenance and production standpoint
Accordingly, it is an objective of the present invention to create a more thermally stable CVD process, utilizing available CVD systems.
To that end, it is another objective of the present invention to prevent overheating and thermal runaway of heated CVD chucks and the malfunctions and problems caused thereby.
It is still another objective of the present invention to eliminate the difficulty of calibrating and verifying the operation of CVD chuck temperature control systems and the RTD probes associated therewith and thus reduce maintenance and production costs.
It is still another objective of the invention to isolate the temperature calibration and monitoring steps for each individual heated chuck of a multi-chuck CVD system to eliminate time wasted on checking all the chucks each time the system is used.
It is another objective to provide monitoring and control of an overheated chuck within a multiple chuck CVD system to prevent thermal runaway of the chuck without significantly affecting the overall CVD system.
These objectives and other objectives will become more readily apparent from the further description of the invention below.